Display screen or portion thereof with a transitional graphical user interface



FIG. 1 is a front view of the first image of a display screen or portionthereof with a transitional graphical user interface showing the newdesign.

FIG. 2 is a front view of the second image of the display screen orportion thereof with a transitional graphical user interface showing thenew design; and,

FIG. 3 is a front view of the third image of the display screen orportion thereof with a transitional graphical user interface showing thenew design.

The appearance of the transitional image sequentially transitionsbetween the images shown in FIGS. 1-3. The process or period in whichone image transitions to another forms no part of the claimed design.The broken lines represent the display screen or portion thereof, text,and graphic elements that form no part of the claimed design.

CLAIM The ornamental design for a display screen or portion thereof with a transitional graphical user interface, as shown and described. 